Processes for preparation of ion exchanged crystalline waveguides

ABSTRACT

There is disclosed an optical waveguide comprising a K 1-x  Rb x  TiOmO 4  single crystal substrate, wherein x is from 0 to 1 and M is P or As, having at least one optically smooth surface wherein sufficient K +  and/or Rb +   have been replaced by ions selected from at least one of H +   and NH 4   +   and, optionally, at least one monovalent ion selected from Rb + , Cs + , Tl + , and/or at least one divalent ion selected from Ba +2 , Sr +2 , Ca +2  and Pb +2  to chance the surface index of refraction at least a 0.00025 with respect to the index of refraction of the single crystal substrate. One process disclosed for producing an optical waveguide comprises the steps of contacting at least one optically smooth surface of a single crystal of K 1-x  Rb x  TiOmO 4  with an ion exchange medium capable of supplying said replacement ions, at a temperature of from about 100° C. to about 600° C. for a time sufficient to increase the surface index of refraction at least about 0.00025 with respect to the index of refraction of the single crystal by replacing K +  and/or Rb +   of said single crystal with said ions supplied by the exchange medium, and cooling the resulting crystal. Another process disclosed for producing an optical waveguide comprises the step of applying a DC voltage of from about 20 V per cm of crystal thickness to about 2000 V per cm of crystal thickness across the z-surfaces of a z-cut K 1-x  Rb x  TiOmO 4  single crystal, in the presence of a proton source at a temperature of from about -40° C. to 400° C. for a time sufficient to raise the index of refraction underneath the anode by at least 0.00025 with respect to the index of refraction of the single crystal by replacing K +  and/or Rb +   of said single crystal with H + . A nonlinear optic device using the optical waveguide of the invention is also disclosed.

This is a division of application Ser. No. 07/738,775 filed Aug. 1,1991, now U.S. Pat. No. 5,146,933.

FIELD OF THE INVENTION

This invention relates to optical waveguides and processes for makingthem.

BACKGROUND OF THE INVENTION

For slab or channel waveguides, it is necessary that the materialthrough which the light is propagated have an index of refraction largerthan that of surrounding media. However, in order that the light ispropagated along and is confined to the slab or channel material thereare more stringent requirements. Generally, modes of propagation areclassified into two kinds according to the orientation of the fieldvectors: those with transverse electric fields (TE modes) and those withtransverse magnetic fields (TM modes). The solutions to Maxwell'sequations for these modes for a slab waveguide are well-known (see e.g.,"Optical Waves in Crystals," Yariv, et al., John Wiley & Sons, N.Y.,Chapter 11 (1984); or "Integrated Optics: Theory and Technology,"Hunsperger, Springer-Verlag, Berlin, 16-37 (1982). The number of suchconfined modes depends on the frequency of the light wave, the depth ofthe slab, and the indices of refraction of the three media involved,i.e., that of the substrate or plate material, n_(s), that of the slabwaveguide material, n_(s) +Δn, and that of the material above the topsurface of the slab waveguide, n_(a). For a given frequency, the numberof confined modes increases with increasing slab depth or withincreasing index of refraction of the slab (i.e., with increasing Δn).For an extremely thin slab or an extremely small Δn, no mode isconfined. As the depth of the slab and/or Δn is increased, one modebecomes confined, then another, etc.

The solutions of Maxwell's equations for confined modes for a channelwaveguide are more complicated than for a slab waveguide. For thisreason, only approximate solutions have been obtained (see "IntegratedOptics: Theory and Technology," Hunsperger, Springer-Verlag, Berlin,38-43 (1982)). For a given value of Δn, there are certain minimum valuesfor the depth and width of the channel in order for the channel to beable to confine a mode. These depth and width values are notindependent, i.e., to confine a given mode, wider channels can be lessdeep while narrower channels require a greater depth. Typically, thedepths and widths of approximately square channel waveguides are severaltimes the depth of a slab waveguide. For both slab and channelwaveguides, the index of refraction must be large enough so that atleast one mode is confined.

Another more recently developed form of a waveguide involves periodicmodulation of the refractive index of the waveguide surface, such asthat described in U.S. Pat. No 5,028,107.

There has been considerable interest in providing crystals suitable foruse in the products of optical devices. Potassium Titanyl Phosphate(i.e., KTP) and certain analogs thereof are of particular note. Forexample, U.S. Pat. No. 3,949,323 discloses crystals of compounds havingthe formula MTiO(XO₄), wherein M is at least one of K, Rb, Tl, or NH₄and X is at least one of P or As and wherein X is P when M is NH₄ andnonlinear optical devices and electro-optic modulators which use suchcrystals. U.S. Pat. No. 4,231,838 discloses a process for themanufacture of single crystals of MTiOXO₄, wherein M is K, Rb or Tl andX is P or As, of optical quality and of sufficient size for use innonlinear optical devices, said process comprising the steps of heatingcertain starting ingredients (chosen to be within the region of aternary phase diagram in which the desired crystal MTiOXO₄ product isthe only stable solid phase) to produce MTIOXO₄, and then controllablycooling to crystallize the desired product. Crystals which have mixturesof elements for M and/or X can be grown by the process. U.S. Pat. No.4,305,778 discloses a hydrothermal process for growing single crystalsof MTiOXO₄, wherein M is K or Rb and X is P or As, said processinvolving using as a mineralizing solution an aqueous solution of aglass defined by specified portions of the ternary diagrams for theselected M₂ O/X₂ O₅ /(TiO₂)₂ system.

Methods of modifying KTP and certain analogs thereof to produce opticalwaveguides have also been studied. For example, U.S. Pat. No. 4,740,265and U.S. Pat. No. 4,766,954 disclose a process for producing an opticalwaveguide comprising contacting at least one optically smooth surface ofa single crystal of a K_(1-x) Rb_(x) TiOMO₄ (wherein x is from 0 to 1and M is P or As) with a specified molten salt of at least one of Rb,Cs, and Tl at a temperature of from about 200° C. to about 600° C. for atime sufficient to increase the surface index of refraction at leastabout 0.00025 with respect to the index of refraction of the startingcrystal, and cooling the resulting crystal. U.S. Pat. No 5,028,107describes periodically modifying crystals to produce a waveguide usefulfor wavelength conversion.

M. M. Eddy et al., Inorg. Chem. 27, 1856 (1988) describes preparationand some properties of compounds of the formula NH4TiOPO₄ (NTP), NH₄H(TiOPO₄)₂ (NHTP), H₃ ONH₄ (TiOPO₄)₂ (NOTP), and H₂ (TiOPO₄)₂ inpowdered form.

R.H. Jarman, Solid State Ionics 32/33, 45 (1989) describes ion exchangereactions using KTiOPO₄ and molten salts containing sodium, lithium andhydrogen ions. Partial ion-exchange of KTiOPO₄ in excess molten NaNO₃was achieved. In contrast, Li salts under the same conditions did notaffect ion-exchange, but instead caused decomposition of the KTP latice;and proton exchange in inorganic or organic media was not achieved.

Other crystal systems have also been studied in connection with opticalwaveguides. For example, Wong, SPIE Vol 993 Integrated Optical CircuitEngineering VI, pages 13-25 (1988) reviews a proton exchange processesfor producing proton exchanged LiNbO3 and LiTaO₃ waveguides.

SUMMARY OF THE INVENTION

The present invention provides an optical waveguide comprising a K_(1-x)Rb_(x) TiOMO₄ single crystal substrate, wherein x is from 0 to 1 and Mis P or As, having at least one optically smooth surface whereinsufficient K⁺ and/or Rb⁺ have been replaced by ions selected from atleast one of H⁺ and NH₄ ⁺ and, optionally, at least one monovalent ionselected from Rb⁺ Cs⁺, Tl⁺, and/or at least one divalent ion selectedfrom Ba⁺², Sr⁺², Ca⁺² and Pb⁺² to change in the surface index ofrefraction at least about 0.00025 with respect to the index ofrefraction of the single crystal substrate. The waveguide consists ofregion(s) in the single crystal substrate where K⁺ and/or Rb⁺ have beenreplaced with ions chosen from at least one of H⁺ and NH₄ ⁺, andoptionally at least one monovalent ion selected from Rb⁺ , Cs⁺, Tl⁺,and/or at least one divalent ion selected from Ba²⁺, Sr²⁺, Ca²⁺ andPb⁺². These waveguides may be used for various optical applications suchas wavelength conversion and electrooptic modulation. The invention alsoprovides nonlinear optic devices which are characterized by employing asan optical element, the optical waveguide of this invention.

A process for producing an optical waveguide in accordance with thisinvention comprises the steps of contacting at least one opticallysmooth surface of a single crystal of K_(1-x) Rb_(x) TiOMO₄ wherein x isfrom 0 to 1 and M is P or As, with an ion exchange medium, convenientlya liquid, capable of supplying ions including at least one of H⁺ or NH₄⁺ and, optionally, at least one monovalent ion selected from Rb⁺, Cs⁺,Tl⁺, and/or at least one divalent ion selected from Ba⁺², Sr⁺², Ca⁺² andPb⁺² at a temperature of from about 100° C. to about 600° C. for a timesufficient to increase the surface index of refraction at least about0.00025 with respect to the index of refraction of the single crystal;and cooling the resulting crystal.

Another process for producing an optical waveguide in accordance withthis invention comprises the step of applying a DC voltage of from about20 V per cm of crystal thickness to about 2000 V per cm of crystalthickness across the z-surfaces of a z-cut K_(1-x) Rb_(x) TiOMO₄ singlecrystal, wherein x is from 0 to 1 and M is P or As, in the presence of aproton source at a temperature of from about -40° C. to 400° C. for atime sufficient to raise the index of refraction underneath the anode byat least 0.00025 with respect to the index of refraction of the singlecrystal by replacing K⁺ and/or Rb⁺ of said single crystal with H⁺.

The relative amounts of NH₄ ⁺ and H⁺ in both processes may be adjustedby heating the guides in the presence of specified partial pressures ofammonia.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of an apparatus for wavelength conversionin accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides an optical waveguide wherein a K_(1-x) Rb_(x)TiOMO₄ single crystal substrate (wherein x is from 0 to 1 and M is P orAs) is modified by replacing sufficient monovalent cations (e.g., K⁺ orRb⁺) in at least one region of the crystal to change the surface indexof refraction at least 0.00025 with respect to the unmodified singlecrystal (i.e., the substrate). In accordance with this invention themonovalent cations are at least partially replaced by H⁺ and/or NH₄ +.Optionally, a portion of the substrate cations may also be replaced byRb⁺, Cs⁺ and/or Tl⁺ ions. Also, a portion of the substrate cations maybe replaced by Ba⁺², Sr⁺², Ca⁺² and/or Pb⁺² ions. Waveguides provided inaccordance with this invention may be in a number of forms, includingchannel waveguides and slab waveguides. Periodically modulated guidessuch as those disclosed in U.S. Pat. No. 5,028,107 (but using H⁺ and/orNH₄ ⁺ as exchange ions) can also be produced in accordance with thespecifications taught therein by masking the single crystal substrate inan appropriate predetermined pattern prior to subjection of the singlecrystal substrate to the ion exchange medium.

Suitable single crystal substrates of the formula K_(1-x) Rb_(x) TiOMO₄can be prepared by known processes (see, for example, U.S. Pat. Nos.3,949,323, 4,231,838 and 4,305,778). The preferred single crystalsubstrate is KTiOPO₄ (i.e., KTP).

The ion exchange medium useful in the practice of this invention ispreferably in the form of a liquid. The characteristics of the liquid tobe used in the process are (a) that it should be substantially stable ata temperature sufficient to cause exchange in the crystal, (b) that isshould not grossly dissolve or etch the surface of the single crystalsubstrate, (c) that it have some solubility for either H⁺ or NH₄ ⁺, and(d) that it be capable of dissolving, i.e., removing from surface, thepotassium or rubidium ions from the single crystal substrate.

Preferred ion exchange liquids for introducing NH₄ ⁺ into the singlecrystal substrate include liquid NH₄ Cl and liquid NH₄ NO₃. Preferredion exchange liquids for introducing H⁺ into the single crystalsubstrate include liquid benzoic and phthalic acids; liquid chloridessuch as GaCl₃, ZnCl₂, BiCl₃, NH₄ Cl; liquid NH₄ NO₃ ; and liquid RbCF₃SO₃ In most cases when salts are used a proton source (e.g., water) isalso present. For example, protons may be derived from water present insalts before melting, water adsorbed from the atmosphere by the moltensalt, or from hydrochloric acid deliberately introduced into the melt.In the case of benzoic and phthalic acids, protons for exchange arederived directly from the acidic functionality of the organic compound,or by bubbling the melt with hydrochloric acid vapor When H⁺ is thedesired exchange ion, the preferred liquid is RbCl/ZnCl₂ which has HCldissolved in it. When NH₄ ⁺ is the desired exchange ion, the preferredliquid is NH₄ Cl/ZnCl₂ which has ammonia bubbled through it.

The monovalent ions Cs⁺, Tl⁺, and Rb⁺ may also be included in the ionexchange medium to give mixed waveguides. Divalent ions Ba²⁺, Sr²⁺, Ca²⁺and Pb²⁺ can also be added to the ion exchange medium to accelerate therate of waveguide formation, provide for domain inversion or allowoperation at a lower temperature. Accordingly, a process for producingan optical waveguide is provided in accordance with this invention whichcomprises the steps of contacting at least one optically smooth surfaceof a single crystal of K_(1-x) Rb_(x) TiOMO₄ wherein x is from 0 to 1and M is P or As, with an ion exchange medium, capable of supplying ionsincluding at least one of H⁺ or NH₄ ⁺ and, optionally, at least onemonovalent ion selected from Rb⁺, Cs⁺, Tl⁺, and/or at least one divalention selected from Ba⁺², Sr⁺², Ca⁺² and Pb⁺², at a temperature of fromabout 100° C. to about 600° C. for a time sufficient to increase thesurface index of refraction at least about 0.00025 with respect to theindex of refraction of the single crystal by replacing K⁺ and/or Rb⁺ ofsaid single crystal with said ions supplied by the exchange medium andcooling the resulting crystal.

Since molten metal chlorides are commonly found to contain dissolvedwater in unpredictable amounts, it may be preferable to eliminate thewater to exercise more exact control over the amount of proton sourcepresent during the process of this invention. Furthermore, the water mayhydrolyze the metal chloride, leading to the production of hydrochloricacid and metal oxide or metal hydroxide. One method of removing thiswater is to bubble the melt with relative dry hydrogen chloride; thisreverses the hydrolysis reaction, regenerating water and metal chloride.The water inevitably has some vapor pressure and is slowly removed fromthe solution along with the flow of hydrogen chloride. If such a dry,HCl-saturated melt is now bubbled with dry nitrogen, a melt containingrelatively less amount of water and protons results. The metal chloridemelt treated in this way can then be used in the process of thisinvention with a controlled amount of proton source.

Another process for producing an optical waveguide is provided inaccordance with this invention which comprises the step of applying a DCvoltage of from about 20 V per cm of crystal thickness to about 2000 Vper cm of crystal thickness across the z-surfaces of a z-cut K_(1-x)Rb_(x) TiOMO₄ single crystal, wherein x is from 0 to 1 and M is P or As,in the presence of a proton source at a temperature of from about -40°C. to 400° C. for a time sufficient to raise the index of refractionunderneath the anode by at least 0.00025 with respect to the index ofrefraction of the single crystal by replacing K⁺ and/or Rb⁺ of saidsingle crystal with H⁺.

For this alternative process for preparing the waveguides of thisinvention, a z-cut single crystal of K_(1-x) Rb_(x) TiOMO₄, preferablyKTiOPO₄, may be provided with conductive electrodes (preferablysputtered gold or sputtered gold-palladium) on both z surfaces. A DCvoltage is applied across the electrodes to give a ratio of appliedvoltage to crystal thickness (measured along the z-axis) from about 20V/cm to about 2000 V/cm at a temperature from about -40° C. to 400° C.for a time sufficient to raise the index of refraction underneath theanode by at least 0.00025. Preferably one uses a ratio of voltage tocrystal thickness of about 300 V/cm at a temperature of about 200° C.The electrodes may be mechanically removed by light abrasion and rubbingor can be removed by chemical means. The waveguide is formed underneaththe anode, or positive electrode. The source of protons for theseelectric field-induced guides is preferably the ambient water in theatmosphere, water explicitly added to the atmosphere during the electricfield treatment, or protons already present in the crystal from itsgrowth. The crystals may also be damaged by a darkening phenomena whichinitially begins at the cathode. The treatment time and temperatureshould be reduced if such electrochromic effect is damaging the crystal.

When an NH₄ ⁺ -exchanged K_(1-x) Rb_(x) TiOMO₄ waveguide of theinvention is heated in the presence of a low partial pressure of ammonia(e.g., air or vacuum), ammonia may be lost with the formation of a H⁺-exchanged waveguide. Conversely, when a H⁺ -exchanged waveguide isheated in the presence of a large partial pressure of ammonia (e.g., 1atm), the protons may be reammoniated to form an NH₄ ⁺ -exchanged guide.In the case of reammoniation, the wave guide often diffuses to a greaterdepth; the resulting index of refraction increase is lower, but theconversion of H⁺ to NH₄ ⁺ leads to a lower value of the relativebirefringence (Δn_(TMO-)Δn_(TEO))/Δn_(TMO). See Examples 7, 8, 10 and10A below. In addition to characterization of these changes throughmeasurement of the effective mode indices, the process may also beobserved through infrared light absorption. The ammonium exchangedguides show absorptions bands at 3175 cm⁻¹ and 3019 cm⁻¹ due to ammoniumion. Deammoniation by heating in a low partial pressure causes adecrease in the intensity of the ammonium bands, and an increase in theOH absorption bands near 3587 cm.sup. -1 and 3566 cm⁻¹. Reammoniation byexposure to ammonia gas causes a decrease in the OH bands, and thereappearance of absorption bands due to ammonium ion.

If too deep excessive a diffusion of protons alone is created, thecrystal may crack. The preferred depth of exchange is 0.5 to about 2.5μm.

The change in refractive index using ammonium exchange and/or protonexchange is different than the change in refractive index usingcomparable amount of rubidium, cesium and/or thalium exchange. Theprofiles in depth of the index of refraction arising from protons arequite different from the depth profile arising from ammonium or rubidiumexchange. The latter two ions give a concave distribution which can befit to an error or exponential function. Protons form a more step likeindex step, and a depth in the substrate is reached where the protonconcentration drops very rapidly from a high and relatively uniformvalue. In addition the increase in refractive index along the z-axisachieved by proton exchange is generally greater than the increase inrefractive index normal to the z-axis (i.e., along the x and y axes).

Proton exchange also differs from Rb exchange in that the protonexchange can diffuse along the y and x directions more easily than canthe Rb exchange. Consequently, there can be 1 undercutting` of the maskin a proton exchange, and the sidewalls need not be so straight.Moreover, waveguides can be more easily fabricated from x and y cutcrystals using proton exchange.

The waveguide of this invention can be used in the appropriate form(e.g., channel, slab, or periodic) as the optical element in a widevariety of nonlinear optic devices. A specific employment of a waveguidein accordance with this invention is illustrated by reference to theapparatus (10) shown in FIG. 1 wherein optical waves emitted by laser(12) at one wavelength may be used to generate waves at anotherwavelength Lens (13) is used to focus the optical waves emitted by laser(12) into a waveguide (14) constructed in accordance with thisinvention. Wavelength conversion occurs within the waveguide (14) and asecond lens (17) is provided to collimate the optical waves emergingfrom the waveguide. A filter (18) is provided in the arrangement shownto filter out the remaining optical waves which have the wavelength ofthe emitted waves, while allowing optical waves of the desiredwavelength which were generated within the waveguide (14) to passthrough. Thus, for example, if laser (12) is a Nd YAG laser used togenerate polarized light at wavelength 1.06 μm, and the waveguide (14)is constructed in accordance with this invention for second harmonicgeneration using such incident light, the filter (18) would be adaptedto allow optical waves of wavelength 0.53 μm to pass through whileoptical waves of wavelength 1.06 μm are filtered from the collimatedbeam which emerges from the waveguide. A device incorporating theapparatus of FIG. 1 (i.e., the laser (12), the waveguide (14), thefilter (18), and the lenses (13) and (17) is considered to be an articlewithin the scope of this invention, as well as the waveguides themselvesand the processes for preparation thereof.

Practice of the invention will become further apparent from thefollowing non-limiting examples.

EXAMPLES

The invention is further illustrated by the following examples in whichall percentages and parts are on a mole bases and temperatures are indegrees Celsius unless otherwise stated. Unless stated otherwise, theeffective indices of refraction of the slab waveguides are determined bythe method of Tien et al., Applied Physics Letters 14, 291-294 (1969) byexamining the samples for waveguiding by the m-line prism-layer couplertechnique described in that article using a 45°-45°-90° rutile prism anda He-Ne laser. When several modes are present, an estimate of the indexof refraction of the surface (Δn surf.) and the depth profile of theindex increase is made using the technique described in J.M. White,Appl. Optics, 15, 151 (1976). Waveguide thickness was determinedoptically for some examples by using a phase contrast microscope, aZeiss Axiomat TM. Waveguide thicknesses measured optically correlatewith those determined by the m-line prism-layer coupler technique. TheΔn for the zero order mode is listed for each example in Table 1 (Δn0-order mode). Depths (d) are provided for some samples.

EXAMPLES 1

A molten salt bath was prepared by heating 50% ZnCL₂ /50% RbCl in aplatinum crucible at 350° C. for about 16 hours. A slab waveguide wasprepared by immersing a KTIOPO₄ (KTP) crystal with a polished z-surfaceinto the melt for 2 hours. The resulting crystal was cooled to roomtemperature, and then was washed with water to remove excess chloridesalts.

The resulting waveguide was then examined by the m-line prism-layercoupler technique and 5 TE modes and 6 TM modes were observed. Theresulting 0-order Δn is characteristic of proton exchange. The resultsare summarized in Table 1.

EXAMPLE 2

A z-cut KTP crystal had gold/palladium electrodes sputtered on to thez-surfaces. A D.C. electric field of 200 V was applied for 1 hour at 30°C. The electrodes were removed by light abrasion and rubbing. A slabwaveguide was created on the anode surface, having 4 modes TE, 5 modesTM. The resulting 0-order Δn is characteristic of proton exchange.

EXAMPLE 3

A salt bath of 50% NH₄ Cl/50% ZnCl₂ was prepared at 325° C. A slow flowof ammonia gas at 1 atm. pressure was bubbled through the melt for 16hrs. Then a z-cut KTP crystal was immersed in the bath for 16 hours withcontinuing flow of ammonia, yielding a planar guide with Δn_(TE) =0.046and Δn_(TM) =0.040. The birefringence is characteristic of an ammoniumguide.

EXAMPLE 4

About 40 g of phthalic acid is placed in a glass tube which is loweredinto a furnace held at 275° C. A mixture of 50 vol % HCl and 50% N₂ isbubbled through the melt at about 2.5 sccm for several hours. A z-cutKTP crystal is immersed in the melt for 270 minutes, removed, cleaned,and prism coupled. Three modes each TE and TM are obtained, the inducedbirefringence being characteristic of a proton guide.

EXAMPLE 5

A salt bath of about 0.035 moles total containing 98% BiCl₃ /2% KCl wasprepared at 300° C. and a mixture of 50% HCl/50% N₂ was bubbled throughthe melt. A z-cut KTP crystal was immersed in the bath for 30 minutes.The crystal was removed and cleaned, and prism coupled--indicating atleast one mode TM. The crystal surface is also etched and made slightlytan color. The birefringence is considered characteristic of protonexchange.

EXAMPLE 6

A salt bath containing 50% RbCl/50% ZnCl₂ was prepared at 300° C. Anx-cut KTP crystal was immersed in the bath for 120 minutes, removed,cleaned, and prism coupled. Two modes TE and two modes TM were observed,with Δn_(TE) =Δn_(z) =0.025 and Δn_(TM) =Δn_(x) =0.017. Thebirefringence is characteristic of the presence of protons in the guide,with Δn_(z) >Δn_(y), Δn_(x).

EXAMPLE 7

A z-cut KTP crystal was immersed for 300 minutes in a salt bath preparedby heating ammonium nitrate in a covered crucible at 200° C. Afterremoval and cleaning, the crystal showed one mode each TE and TM, withΔn_(TE) =0.009 and Δn_(TM) =0.0006. The birefringence is characteristicof ammonium exchange.

EXAMPLE 8

The sample from Example 7 was annealed at 250° C. in air for 60 minutes.The result was a slab guide with one mode each TE and TM, but Δn_(TE)=0.012 and Δn_(TM) =0.032. The annealing resulted in loss of ammoniafrom ammonium ions, resulting in a proton guide with its characteristicincreased birefringence.

EXAMPLE 9

A melt was prepared by placing solid GaCl₃ into a glass tube and heatingto 140° C. It was held at 140° C. for several hours, and atmosphericmoisture was allowed to come into contact with the liquid. A z-cut KTPsample was immersed in the bath for 4 hours. Upon removal, cooling,cleaning, and prism coupling, a slab waveguide resulted with 3 modes TM.The increase in the index along of n_(z) is larger than that of n_(y) orn_(x), which is characteristic of proton exchange.

EXAMPLE 10

A melt of a total of about 0.035 moles of 50% NH₄ Cl/50% ZnCl₂ wasprepared at 300° C. in a covered quartz crucible. A z-cut KTP crystal isimmersed in the melt for 15 minutes. After cleaning, prism couplingshows 4 modes each TE and TM. The size of the index increase and thebirefringence is characteristic of a guide containing both ammonium andprotons.

EXAMPLE 10A

The guide from Example 10 was ammoniated by placing the crystal in aPyrex tube containing NH3 at 1 atm. pressure for 60 min at 300° C. Thesurface index is reduced by ammoniation from Δn_(TMsurf) =0.072 to0.045. The reduction in birefringence and lowering of the index ofrefraction is consistent with the transformation of a substantialfraction of the protons to ammonium ion.

EXAMPLE 11

A melt was prepared from about 1g of RbCP₃ SO₃ at 325° C. The melt wasbubbled with a gas stream consisting of a mixture of 1 part N₂ which hadbeen equilibrated with water at room temperature and 2 parts dry HCl. Az-cut KTP crystal was immersed in the melt for 10 minutes. After removaland prism coupling, 3 modes TE and 4 modes TM were observed. Δn_(TMO)-Δn_(TEO) is 0.023. The birefringence is characteristic of protonexchange.

EXAMPLE 12

About 37 g of benzoic acid was melted in a glass tube at 220° C. andheld at this temperature for 1 day. A z-cut KTP crystal was immersed inthe liquid for 3 days at 220° C. After removal and prism coupling, aslab waveguide was produced having 2 modes TE and 2 modes TM. Thebirefringence is characteristic of proton exchange.

EXAMPLE 13

A melt containing RbCl/ZnCl₂ /BaCl₂ in the percentage 73/24/3 wasprepared in a glass beaker at 300° C. A z-cut KTP crystal was immersedin the melt for 120 min, removed, cooled, cleaned. Prism couplingindicated 9 modes TE and 13 modes TM. The index-depth profile shows aregion from the surface to a depth of about 4 mm which has a highinduced birefringence (Δn_(TM) -Δn_(TE) ˜0.027). This region isrelatively rich in protons relative to rubidium. A `tail` in theindex-depth profile is a lower index region extending to a depth ofabout 20 μm characterized by a lower increase in index and less inducedbirefringence (both on an absolute and a relative basis) This region isthought to have more Rb relative to H. Comparison of this Example withthat of Example 1 shows well the effect of the divalent ion Ba.

EXAMPLE 14

A melt of ZnCl₂ was prepared at 325° C. in a glass tube and a slow flowof HCl was bubbled through the melt for 20 hours. A z-cut KTP crystalwas immersed in the bath for 20 min at 325° C. Upon removal and prismcoupling, 4 modes TE and 5 modes TM were observed. This is an example ofa waveguide in which most of the exchanged ions are protons. Its indicesshould be comparable with those of Examples 4, 5, and 8.

EXAMPLE 15

A z-cut KTP crystal, of thickness about 1 mm along the z direction hadgold electrodes sputtered on to the z-surfaces. The crystal was grown bythe hydrothermal method at a growth temperature of 600° C. The crystalwas placed in a chamber at 200° C. A slow flow of air was bubbledthrough water at 95° C., and then through a heated tube into the chambercontaining the crystal. This provided an atmosphere in the chamberhaving approximately 0.8 atm water vapor and the remainder air. Thecrystal was held in the chamber for 160 min., and then a DC voltage of30 V was applied across the electrodes for 20 minutes. The crystal wasimmediately removed from the chamber, and the gold electrodes removed bylight abrasion. A slab waveguide was created on the anode surface,having 1 mode each TE and TM. The birefringence is characteristic ofproton exchange.

EXAMPLE 16

Crystalline Zn(OH)Cl was prepared as described in G. Brauer (ed.)Handbook of Preparative Inorganic Chemistry, Vol. 2 (2nd ed.), AcademicPress (1965) p. 1071. A molten salt bath was prepared by heating ZnCl₂/Zn(OH)Cl 95/5 to 300° C. in a crucible exposed to the air. A z-cut KTPcrystal was immersed in the liquid for 15 min. Upon removal and prismcoupling, 4 modes each TE and TM were observed. (This is an example of aproton guide where at least some protons are explicitly present in theformula of the starting melt.)

EXAMPLE 17

A melt of NH₄ NO₃ was prepared at 200° C., and a z-cut KTP crystal wasimmersed in the melt for 420 min. After removal and prism coupling, nomodes TE or TM were observed.

EXAMPLE 17A

The sample from Example 17 was placed on a hot plate in air for 20 min.at 250° C. One mode each TE and TM resulted. (This is an example of anammonium diffusion which is too shallow to produce a guide. Upondeammoniation, a proton guide appears. So the deammoniation not only canconvert one type into another, it can cause guiding where none existed.)

EXAMPLE 18

A mixture of NH₄ NO₃ /Pb(NO₃)₂ 70/30 was heated to 200° C. in a glasstube. A slurry formed, and presumably not all of the lead is dissolved.A z-cut KTP crystal was immersed in the melt for 1 hour. Upon removaland prism coupling, the guide gave 1 mode TE and one mode TM. Thebirefringence is characteristic of ammonium exchange.

EXAMPLE 19

A melt was prepared from KCl/ZnCl₂ 50/50 at 325° C. A mixture of N₂ /HCl95/5 was bubbled through the melt. A z-cut KTP crystal was immersed inthe melt for 15 min. Upon removal and prism coupling, 1 mode each TE andTM were observed. In comparison with Example 14, this is an example inwhich lower concentrations of HCl and the diluent KCl are used tocontrol the waveguide to a shallower depth. The birefringence ischaracteristic of proton exchange.

                                      TABLE 1                                     __________________________________________________________________________                           No.,                                                                          Type Δn                                              Liquid or Temp.                                                                              Time                                                                              Modes                                                                              O-order                                                                            Δn                                     Ex. Process Used                                                                            (°C.)                                                                       (min)                                                                             Obsd.                                                                              mode surf.                                                                             d(μm)                                 __________________________________________________________________________     1  RbCl/ZnCl.sub.2                                                                         350  120 5 TE 0.014                                                                              0.021                                                                             2.1                                          50/50              6 TM 0.034                                                                              0.044                                         2  D.C. electric                                                                            30  ˜60                                                                         4 TE 0.049                                                 field              5 TM 0.053                                              3  NH.sub.4 Cl/ZnCl.sub.2 50/50                                                            325  960 7 TE 0.032                                                                              0.046                                                                             3.0                                          with 1 atm NH.sub.3                                                                              6 TM 0.027                                                                              0.040                                         4  Phthalic acid                                                                           275  270 3 TE 0.018                                                                              0.025                                                                             2.9                                          with 50/50 HCl/N.sub.2                                                                           3 TM 0.040                                                                              0.048                                         5  KCl/BiCl.sub.3 2/98                                                                     300  30  0 TE --                                                    with 50/50 HCl/N.sub.2                                                                           ≧1 TM                                                                       0.033                                              6  RbCl/ZnCl.sub.2 50/50                                                                   300  120 2 TE 0.025                                                 x surf. exchanged  2 TM 0.017                                              7  NH.sub.4 NO.sub.3                                                                       200  300 1 TE  0.0009                                                                  1 TM  0.0006                                            8  Crystal from Ex.                                                                        250  60  1 TE 0.012                                                 7 annealed in air                                                                       (anneal) 2 TM 0.032                                              9  GaCl.sub.3                                                                              140  240 0 TE --                                                                       3 TM 0.020                                                                              0.030                                        10  NH.sub.4 Cl/ZnCl.sub.2 50/50                                                            300  15  4 TE 0.038                                                                              0.047                                                                             2.8                                                             4 TM 0.059                                                                              0.072                                          .sup. 10A                                                                       Crystal from Ex.                                                                        300  60  5 TE 0.027                                                                              0.034                                                                             3.0                                          10 after           5 TM 0.036                                                                              0.045                                            ammoniation                                                               11  RbCF.sub.3 SO.sub.3 with                                                                325  10  3 TE 0.039                                                                              0.048                                                                             2.4                                          HCl/wet N.sub.2    4 TM 0.062                                                                              0.073                                        12  Benzoic acid                                                                            220  4320                                                                              2 TE 0.019                                                                    2 TM 0.041                                             13  RbCl/ZnCl.sub.2 /BaCl.sub.2                                                             300  120 9 TE 0.019                                                                              0.022                                                                             3.3                                          73/24/3            13 TM                                                                              0.046                                                                              0.053                                        14  ZnCl.sub.2 with HCl                                                                     325  20  4 TE 0.020                                                                              0.025                                                                             4.5                                                             5 TM 0.040                                                                              0.047                                        15  Electric field                                                                          200  20  1 TE 0.003                                                 with H.sub.2 O vapor                                                                             1 TM 0.012                                             16  ZnC1.sub.2 /Zn(OH)Cl                                                                    300  15  4 TE 0.023                                                 95/5               4 TM 0.047                                             17  NH.sub.4 NO.sub.3                                                                       200  420 0 TE --                                                                       0 TM --                                                  .sup. 17A                                                                       Crystal from Ex.                                                                        250  20  1 TE  0.0006                                               17 annealed in     1 TM  0.0082                                               air                                                                       18  NH.sub.4 NO.sub.3 /Pb(NO.sub.3)2                                                        200  60  1 TE 0.014                                                 70/30              1 TM 0.010                                             19  KCl/ZnCl.sub.2                                                                          325  15  1 TE  0.0009                                                                  1 TM  0.0059                                           __________________________________________________________________________

Particular embodiments of the invention are included in the examples.Other embodiments will become apparent to those skilled in the art froma consideration of the specification or practice of the inventiondisclosed herein. It is understood that modifications and variations maybe practiced without departing from the spirit and scope of the novelconcepts of this invention. It is further understood that the inventionis not confined to the particular formulations and examples hereinillustrated, but is embraces such modified forms thereof as come withinthe scope of the following claims.

What is claimed is:
 1. A process for producing an optical waveguidecomprising the steps of: contacting at least one optically smoothsurface of a single crystal of K_(1-x) Rb_(x) TiOMO₄ wherein x is from 0to 1 and M is P or As, with an ion exchange medium including a liquidselected from the group consisting of liquid NJ₄ Cl, liquid NH₄ NO₃,liquid phthalic acid, liquid GaCl₃, liquid ZnCl₂, liquid BiCl₃ andliquid RbCF₃ SOP₃, which supplies (i) ions selected from the groupconsisting of H⁺ and NH₄ ⁺ and, optionally (ii) ions selected from thegroup consisting of Rb⁺, Cs⁺, Tl⁺, Ba⁺², Sr⁺², Ca⁺² and Pb⁺², at atemperature of from about 100° C. to about 600° C. for a time sufficientto increase the surface index of refraction at least about 0.00025 withrespect to the index of refraction of the single crystal by replacingK⁺, Rb⁺ or both K⁺ and Rb⁺ of said single crystal with said ionssupplied by the exchange medium including sufficient ions selected fromthe group consisting of H.sup.° and NH₄ ⁺ to increase the infrared lightadsorption band for OH, NH₄ ⁺ or both OH and NH⁺ ions; and cooling theresulting crystal.
 2. The process of claim 1 wherein the ion exchangemedium is a liquid.
 3. The process of claim 2 wherein NH₄ ⁺ isintroduced into the single crystal substrate, and the ion exchangeliquid includes liquid NH₄ Cl or liquid NH₄ NO₃.
 4. The process of claim3 wherein the ion exchange liquid is NH₄ Cl/ZnCl₂ which has ammoniabubbled through it.
 5. The process of claim 2 H⁺ is introduced into thesingle crystal substrate and the ion exchange liquid includes liquidbenzoic acid, liquid phthalic acid, liquid GaCl₃, liquid ZnCl₂, liquidBiCl₃, liquid NH₄ Cl, liquid NH₄ NO₃ or liquid RbCF₃ SO₃.
 6. The processof claim 5 wherein the ion exchange liquid is RbCl/ZnCl₂ which has HCldissolved in it.
 7. The process of claim 2 wherein the ion exchangemedium includes a monovalent ion selected from the group consisting ofCs⁺, Tl⁺ and Rb⁺.
 8. The process of claim 7 wherein the ion exchangemedium include a divalent ion selected from the group consisting ofBa²⁺, Sr²⁺, Ca²⁺ and Pb²⁺.
 9. A process for producing an opticalwaveguide comprising the step of applying a DC voltage of from about 20V per cm of crystal thickness to about 2000 V per cm of crystalthickness across the z-surfaces of a z-cut K_(1-x) Rb_(x) TiOMO₄ singlecrystal, wherein x is from 0 to 1 and M is P or As, in the presence of aproton source at a temperature of from about -40° C. to 400° C. for atime sufficient to raise the index of refraction underneath the anode byat least 0.00025 with respect to the index of refraction of the singlecrystal by replacing K⁺, Rb⁺ or both K⁺ and Rb⁺ of said single crystalwith H⁺.
 10. The process of claim 9 wherein the ration of voltage tocrystal thickness is about 300 V/cm, and the temperature is about 200°C.
 11. The process of claim 9 wherein waveguide is heated in thepresence of sufficient ammonia partial pressure to incorporate ammoniainto said guide by ammoniating said H⁺.
 12. The process of claim 9wherein the proton source is ambient water in the atmosphere, waterexplicitly added to the atmosphere or protons already present in thecrystal as a result of its growth.